Building exhaust system: muffler placement relative to a turbo?

For my Turbo-1 swapped Rampage, I'm working on putting together a more 'real' exhaust for it. Up until now it's either run with just the downpipe cut just before where the cat would have been or a 90-degree angle and straight pipe out under the side of the car to comply with the Challenge rules regarding where the exhaust has to exit.

I grabbed one half of the exhaust system off of a newer V8 Ram truck from the junkyard which includes a muffler and good bit of 2.25" OD exhaust pipe. I've worked out, in general, how to use it to make the exhaust system but I'm curious about whether I can put the muffler where it looks most convenient- right after the downpipe- or whether I need to try and push it further back with a few feet of pipe between the downpipe and the muffler so that it doesn't interfere with the exhaust flow from the turbo.

 Essentially, can I put the muffler where the cat was without messing with the turbo, or does it need to be further away. It's not going to be THAT far back though, will still be in the tunnel under the center of the truck with the pipe running out to dump out the side behind the driver's door. Thanks!

We're talking a couple feet after the turbine here, right?  I don't think you'll have an issue unless it's literally so close that the O2 sensor is right up against the muffler inlet or something.

Also, turbos make good mufflers and the challenge has no decibel limit, so why bother?

2.25"?  I'd keep looking for 2.5" stuff, like off a V6 minivan or take off 3" stuff from a diesel truck.  Just me though.

That said, in this case it doesn't matter where in the system the muffler is other than the closer to the turbo outlet, the hotter the exhaust will be, so the muffler may wear out slightly faster if its a glasspack or similar.

Anything after the turbo is just a restriction, so the less restriction, the better for performance.

This might help with later designs: http://gusmahon.org/html/mufflers.htm

I've always felt that a turbo makes an ideal muffler all by itself.  Even so, I'm running a 3" Borla XR-1 on my turbo FC just to quiet things down even more.

Benefit of downstream distance would be cooler exhaust, if the muffler is of a sort to be burning up.  Also, hotter exhaust = greater volume = greater restriction.  Conversely, cooler exhaust = lesser volume = lesser restriction.

The turbo is a muffler.

Turbo -> downpipe -> high-flow cat -> resonator -> out the back/side/roof

Basically, you want the exhaust to flow as freely as possible in order for the turbo to be able to spin as easily as possible.  The whole thing works because the pressure on the head side of the turbo is greater than the pressure on the downpipe side.  So unlike an NA car where you need to consider scavenging pulses and stuff, that's all happening inside the manifold before the turbo.  You want a huge downpipe so that there is plenty of volume for the expanding gasses to fill without putting any reversing pressure on the turbine.  If you need to put a restriction in (muffler/resonator/crazy bend/smaller pipe), you put that as far away as possible so the gasses have time to cool down and shrink, but the goal is to put as few restrictions as possible.

You need a 3" downpipe, put a 3" high flow cat in there, and then run it out the back.  If you don't like the sound, you can add a resonator to it, but most of the sound elimination is going to happen via the turbine housing itself.  The cat will help as well.  If you absolutely must put a muffler on the system for whatever reason, I would put it at the very back of the exhaust so everything has a chance to cool down. 

And ditch the pipe you've got.  Two and a quarter is just too small.  Anything bigger than about 3 inch is a little hard to maneuver around the underside (heh heh), but turbos need the free-est exhaust that you can give them in order to be able to really work.  If you're not letting them breathe, you're going to generate nothing but heat.

I'd say to run the exhaust out the back to limit driver fatigue, it also helps a driver hear more of what the car is doing without the drone.

The downpipe off the turbo is only 2.5" OD, so I don't imagine I'd see much benefit of going any larger than that for the rest of the system. I guess for now I'll put together a 'street' exhaust for it with what I have and see about finding larger pipe for a race setup down the line. For the street setup I definitely want the muffler- if nothing else SWMBO and our dog will appreciate it.

In reply to wae :

None of the systems are going to have a cat- KY doesn't have emissions inspections to worry about and it didn't have one when I got it so I'm not going to be putting one in unless I need to.

Just as a reference point, here's what I welded up for the Neon.  I bought a 3" downpipe with a flex section from eBay just so I didn't have to deal with the flanges and making sure I got the bend all correct.  I went through Jegs to get the resonator and a couple straight sections of 3" pipe and a couple U-bends that I could cut and weld together.  I sized it up under the car and then welded in the driveway:

From the left you have the pipe that connects to the downpipe via flex and clamps, then the resonator, a bend around the gas tank, and then up over the suspension and out the back.

Here's the connection to the downpipe:

And the resonator tucked right in the middle of the car:

With the new motor installed, it sounded pretty nice:

https://youtu.be/1cwrdcCJYy4?t=37s

If you can take the noise, the performance advantage of a screamer pipe setup (short straight-pipe dump from the wastegate) is enormous. Or to put it another way, any backpressure after the wastegate is seriously bad for performance.

You could probably run the screamer pipe through a separate straight through muffler and cut the noise a lot but retain most of the performance gain.  AFAIK, not merging it back into the main exhaust flow is the big thing.  

In reply to GameboyRMH and rslifkin :

Do you guys have sources for this stuff?  I'm having trouble finding anything with back to back comparisons.

I remember that what really opened my eyes to this effect was a MotoIQ article that did a back to back comparison. Their site is giving 502 errors right now though.

Ashyukun said:

The downpipe off the turbo is only 2.5" OD, so I don't imagine I'd see much benefit of going any larger than that for the rest of the system. I guess for now I'll put together a 'street' exhaust for it with what I have and see about finding larger pipe for a race setup down the line. For the street setup I definitely want the muffler- if nothing else SWMBO and our dog will appreciate it.

You're wrong on that point.  Going less than 2.5" on that motor, even with the small turbo it currently has, will cost you some power.

With Turbo motors you have to throw the regular school of thought about exhausts out, just a fact of life.

That doesn't mean you can't have a decent noise level though and since you need something, go ahead and use what you have for now.  Just know that you really should look for larger piping and a higher flowing muffler down the road and you may find that routing the exhaust out the back will improve the noise levels in the cabin.  Either way, you'll want the exit to be past the edge of the body to cut down on noise in the cabin and with a side exit that can look a little funny.

Carroll Shelby's personal Shelby Charger had modified side skirts that integrated square exhaust tips into it.  Looked very cool, but I can't find a picture at the moment.  Since your skirts are too short (giggity) that might be an option to look at.  From what I remember, one side was a dummy, likely the passenger side since that is where the fuel filter and lines are.  Would probably help concourse scores a bit, especially if you add the historical fact into the presentation.

rslifkin said:

You could probably run the screamer pipe through a separate straight through muffler and cut the noise a lot but retain most of the performance gain.  AFAIK, not merging it back into the main exhaust flow is the big thing.  

Wastegate is integrated into the exhaust housing of the turbo on these motors, a screamer pipe isn't possible without changing to a turbo with an external wastegate solution.  That will be hard to do within the budget confines and the confines of that engine bay.

Ashyukun said:

The downpipe off the turbo is only 2.5" OD, so I don't imagine I'd see much benefit of going any larger than that for the rest of the system. I guess for now I'll put together a 'street' exhaust for it with what I have and see about finding larger pipe for a race setup down the line. For the street setup I definitely want the muffler- if nothing else SWMBO and our dog will appreciate it.

Necking that down to 2 and a quarter when those gasses are still hot and all expand-y is going to hurt power, build heat, and cook oil.  Even if you don't do the downpipe now, having 3" after the DP will help those gasses by giving them a place to go and start cooling which will drop the pressures and heat.